1. Field of the Invention
The present invention relates to a visual line detection device which detects the direction of the point of attention towards which an observer is directing his attention within a field of view provided for example within an optical device, i.e., which detects the direction of the so called visual line, and also relates to a camera equipped with such a visual line detection device.
2. Description of Related Art
In art relating to devices for detecting the visual line of an observer, for example, there is a type of visual line detection device disclosed in Japanese Patent Laid Open Publication Heisei 3-109029, which it is not hereby intended to admit as prior art to the present application except to the extent in any case mandated by applicable law. With the device disclosed in this publication, the eyeball of the observer is illuminated by a light source. The central position of the pupil is determined by sensing the boundary between the pupil and the iris of the eyeball illuminated by this light source; The position of the image of the light source generated by the light rays from the light source and reflected from the cornea is determined. The visual line is determined from the relative positional relation of this pupil central position and the image reflected from the cornea. In the following, the operation of the visual line detection device according to the above identified publication will be explained with reference to FIG. 30.
FIG. 30 is a sectional view taken through a human eyeball in a horizontal plane. In this figure, the reference numeral 1 denotes the eyeball, and this eyeball 1 generally consists of a sclera 2 of generally spherical shape filled with a vitreous body 3, with a lens 4, an iris 5, and a cornea 6 being formed in the front portion of this sclera 2 (the upper left portion in the figure). The iris 5 is a membrane surrounding a central open portion or aperture 7 which is called the pupil, and said iris 5 includes muscles which can be contracted or relaxed so as to open or close said pupil 7. The curvature of the cornea 6 is substantially greater than the curvature of the sclera 2 and of the vitreous body 3 contained therein. If, as shown in the figure, the distance between the center of curvature C of the cornea and the center of rotation O' of the eyeball 1 is called .rho., and the distance between the center D of the pupil and the center of rotation O' of the eyeball is called A, then, although these distances do in fact vary slightly between individuals, they can be assumed to be almost constant in practice.
As shown in FIG. 30, the X axis is taken in a horizontal direction (the up and down direction in the figure), and the position along the X axis of the center of rotation O' of the eyeball when the eyeball 1 is applied squarely against the center of a plane of view not shown in the figure (and is thus in a different position, both angularly and in parallel displacement along the X axis, from its shown position) will be taken as the origin O. The position on the X axis of the center D of the pupil (this is also designated by the symbol D) and the position on the X axis of the image P reflected from the cornea (this is also designated by the symbol P) are given by, if the angle .theta. of rotation of the eyeball 1 is defined as shown in the figure: EQU D=L+A.times.sin.theta. (1) EQU P=L+.rho..times.sin.theta. (2)
Here, the image P reflected from the cornea is what is called the first Purkinje image, and is a virtual image created by light rays reflected from the surface of the cornea, when the cornea 6 is considered as a convex lens.
By substituting from equation (1) into equation (2), it is possible to eliminate the component L of parallel displacement of the center of rotation O' of the eyeball, and EQU D-P=(A-.rho.).times.sin.theta. (3)
is obtained. At this time, if it is supposed that the angle .theta. of rotation of the eyeball 1 is small, then, by postulating that sin .theta. is approximately equal to .theta., the angle .theta. of rotation of the eyeball 1 can be calculated by rewriting the above equation (3) to give: ##EQU1##
Since the distance D-P can be measured from the image reflected from the eyeball 1, therefore the angle .theta. of rotation of the eyeball 1 can be calculated by substituting an appropriate constant for the distance A-.rho..
Nevertheless, with the visual line detection device disclosed in the above described publication, a problem arises in connection with the fact that the equations (1) through (3) essentially only hold for a reflected image from the cornea focused from parallel light rays. Because in practice the calculations for visual line detection are performed using these equations (1) to (3) for a light source for visual line detection which is at a finite distance and not at infinity, the problem arises of a risk that deviation of the obtained visual line from the actual visual line of the observer can occur. This will be explained with reference to FIGS. 31(A)-31(C).
FIG. 31(A) shows the case where parallel light rays PL from a light source S at infinity are squarely incident upon the front face of the eyeball 1, and the image P reflected from the cornea 6 is generated at a position at a distance exactly r/2 towards the light source (at infinity) from the center C of curvature of the cornea (r is the radius of curvature of the cornea 6); and moreover the center O' of rotation of the eyeball 1, the center C of curvature of the cornea 6, and the image P reflected from the cornea 6 are all collinear. If however this light source S is positioned at a finite distance as shown in FIG. 31(B), then due to the fact that the light rays from this light source S at a finite distance are diverging light rays DL, the image P' reflected from the cornea 6 is generated at a position displaced exactly Ar towards the light source S, as compared to the above described case of parallel light rays PL. Further, if as shown in FIG. 31(C) the eyeball 1 is displaced in parallel in the downwards direction as seen in the figure by exactly a distance L, then the image P' reflected from the cornea 6 is generated at a position displaced angularly around the center C of curvature of the cornea 6 by exactly an angle .alpha., displaced .DELTA.r' towards the light source, and as seen from the front is displaced by exactly .DELTA.L in the upwards direction as seen in the figure. On the other hand, if parallel light rays PL were to be incident squarely upon the front face of the eyeball 1, even if as shown in FIG. 31(C) the eyeball 1 were subjected to parallel displacement, the image reflected from the cornea 6 would still be generated at the position P'. Accordingly, .DELTA.L is the amount of deviation due to the light source S being positioned at a finite distance.